1. Field of the Invention
The present invention relates to minimizing simultaneous transmissions and efficiently utilizing a multiple access communications channel. More particularly, the invention concerns determining whether a user should transmit a signal over a multiple access communications channel during a particular time slot.
2. Description of Related Art
In many applications communications channels are shared by multiple users. Use of a communications channel by multiple users must be coordinated to prevent excessive interference between users' transmissions. Various methods, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and Code Division Multiple Access (CDMA), have been devised to permit multiple users to share a communications channel. TDMA is used in many applications.
With TDMA, a communications channel is shared by defining time slots, and permitting preferably only one user to transmit during any particular time slot. Generally, it is desirable to permit each user to transmit their messages with minimal delay from the time that they have a message to transmit. It is also desirable to minimize collisions that result from simultaneous transmissions from multiple users onto the same channel, because the simultaneously transmitted signals can become garbled and difficult or impossible to receive.
Protocols referred to as multiple access protocols have been devised to arbitrate the use of broadcast channels by multiple users. Dynamic Reservation protocols are an example of a type of multiple access protocol that can be used with TDMA systems. Generally, with a Dynamic Reservation protocol, users transmit channel access requests, which must be coordinated and resolved before a user can transmit a data message. Coordinating and resolving the channel access requests also generally requires transmitting over the channel. Thus, bandwidth is undesirably utilized by the channel access request messages and the messages for resolving channel access requests. Additionally, there can be simultaneous transmissions of channel access requests and messages for resolving access requests. Furthermore, Dynamic Reservation protocols inherently have a delay of a number of cycle times before a message can be transmitted, due to the time required to transmit channel access requests and to coordinate and resolve requests, and the cycle time increases as the number of units increases. Consequently, Dynamic Reservation protocols have disadvantages for many applications.
The Dedicated Access protocol is an example of another TDMA multiple access protocol. The Dedicated Access protocol has been used with Link-16, which is a frequency hopping spread spectrum communications protocol, and which is the primary tactical data link used by the U.S. Department of Defense. Link-16 uses TDMA frames, wherein each frame has 1,536 slots/12 seconds=7.8125 ms/slot. With the Dedicated Access protocol, each user is assigned a fixed set of transmission time slots during which the user may transmit messages. Each user may be allocated the same number of time slots in a frame, or if desired, different numbers of time slots in a frame may be allocated to different users. FIG. 1 shows a representation of a Dedicated Access protocol, in which each user is assigned one time slot. With Dedicated Access, simultaneous transmissions do not occur between users' transmissions because each time slot is assigned to a single user, and because each user can transmit only during their assigned time slots. However, Dedicated Access has a number of disadvantages.
A significant shortcoming of Dedicated Access is that, before the communications channel can be used by a group of users, time slots must be assigned to the users. A network design phase, which typically takes several weeks even for a small number of users, is required to determine the time slot assignments. This long delay is undesirable, and is particularly unsatisfactory during crisis situations. Also, a user's time slot assignments cannot be changed without conducting another network design phase. Another deficiency of Dedicated Access is that users cannot be added or removed from the communications network, because new time slot assignments must be produced in a new network design phase before users can be added or removed.
Additionally, because time slots cannot be reassigned in the Dedicated Access protocol, if one or more users are not present while the communications channel is being used, the time slots assigned to the absent users will not be used and channel capacity will be wasted. As an example, the group of users to which time slots are assigned may include a number of aircraft and ships (which may be called platforms). If one or more of the aircraft or ships are not present, the time slots assigned to those aircraft or ships will be wasted.
Dedicated Access is also inefficient because the number of time slots allocated to users is fixed, whereas the transmission needs of users typically fluctuate randomly. Users' transmission needs typically fluctuate depending on random events, for example events that occur on a battlefield. Consequently, many times the fixed number of time slots allocated to a user with the Dedicated Access protocol are insufficient for the user to disseminate the user's messages quickly, resulting in poor performance. At other times, one or more users may have time slots that they are not using because they do not have a message to transmit. With the Dedicated Access protocol the time slots that are assigned to a user cannot be used by other users, and consequently time slots that are not used by their assigned users remain unused and channel capacity is wasted.
With Dedicated Access, the total number of time slots assigned to users may be increased when the number of users increases, and consequently, the cycle time, which is the sum of all of the time slots, will also increase. Consequently, the waiting time that each user must wait before being able to transmit a message increases. Additionally, when there are a large number of users, the number of time slots assigned to any particular user may decrease, making it more likely that a user will require additional cycle times to complete transmission of a message.
In summary, Dedicated Access is unsatisfactory in many applications because it requires a long network design phase, it cannot support the unplanned entries and exits of users without incurring another network design phase, it cannot efficiently accommodate the fluctuating transmission needs of users, and because it cannot scale to support a large number of users efficiently. Consequently, known protocols for arbitrating multiple users' accesses to many types of communications channels can be improved upon.